The invention relates to a method for making webs and, in particular, to a new method for extrusion coating multiple webs simultaneously on a dual or tandem extruder system having multiple extruder stations.
Webs such as paper and paperboard, polymer films, and metal foils which have been extrusion coated with a layer of polymer film are used in a myriad of applications such as packaging materials and so forth. In some instances, it is desireable to apply two extrusion coatings to a substrate web so that both sides of the substrate are coated with an extruded polymer film. For these cases, so called xe2x80x9ctandemxe2x80x9d or dual extruders have been developed with multiple extrusion stations.
However, cases requiring a coating on both sides of a web by means of a dual extruder may not be plentiful enough to maintain a tandem extruder at full capacity. In these instances, it has been common practice to simply shut down one of the extruder stations and operate the dual extruder in the same manner as a single station extruder. While this prevents the extruder from remaining completely idle, it still represents a significant waste of capital resources.
It is therefore an object of the invention to provide a method for achieving more efficient and economical utilization of a dual or multiple station extruder.
Another object of the invention is to provide a method for producing extrusion-coated substrates at a higher production rate.
With regard to the foregoing and other objects and advantages, the present invention provides a method for extrusion coating multiple webs in an extruder system containing multiple extrusion stations. In one embodiment, the method includes feeding a first web and a second web into a first extrusion station in the extruder system, where each web has a coating side and an underside. The webs are arranged one with respect to the other to form a temporary composite wherein respective undersides of the webs face each other and their respective coating sides face outwardly, away from each other. A first polymer coating is extruded on the coating side of the first web at the first extrusion station. The composite is fed to a second extrusion station following extrusion of the first polymer coating thereon. At the second extrusion station, a second polymer coating is extruded on the coating side of the second web of the composite. The extrusion coated first and second webs are then separated.
According to another aspect of the invention, a method is provided for extrusion coating multiple webs in an extruder system containing multiple extrusion stations which includes feeding a first web and a second web into the extruder system, each web having a coating side, conveying the first web to a first extrusion station in the extruder system and extruding a first polymer coating on the coating side of the first web at the first extrusion station and conveying the second web to a second extrusion station in the extruder system and extruding a second polymer coating on the coating side of the second web at the second extrusion station.
It is also preferred in certain embodiments that the first and second webs comprise the same or similar materials. However, in other embodiments it is preferred that the first and second webs comprise substantially different materials. Preferred materials for the first and second webs include paper webs, polymer films, nonwoven fabrics, metal foils, and two or more thereof as a multi-ply web. The first and second webs preferably each have a basis weight of from about 7 lbs/3000 ft2 to about 125 lbs/3000 ft2 and a caliper of from about 0.5 mils to about 10 mils.
Likewise, in certain embodiments it may be preferred that the first and second webs be extrusion coated with the same type of polymer. However in certain other embodiments, it may be preferred that the first and second webs are extrusion coated with different types of polymers. Preferably the first and second polymer coatings comprise a polymer selected from the group consisting of low density polyethylene (LDPE), polyolefin plastomers (POP), polyolefin elastomers (POE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), ethylene methyl acrylate copolymer (EMA), ethylene butyl acrylate copolymer (EnBA), ethylene vinyl acetate copolymer (EVA), ethylene acrylic acid copolymer (EAA), ethylene methyl acrylic acid copolymer (EMAA), ionomoers such as SURLYN, ethylene vinyl alcohol (EVOH), polyesters such as polyethylene terephthalate (PET), and polyamides (PA) such as nylon-6 and nylon-6,6, and mixtures of two or more thereof.
The invention therefore enables use of a dual station extruder to provide webs which are extrusion coated only on one side. In comparison to prior art, methods, the method of the invention essentially increases the production rate for such single-side coated webs by a factor of two or more.